A combination of injuries and loss of form sent the 24-year-old out of favour at Veltins-Arena, leading to a limitation in game time.
With just four appearances in total for the Royal Blues this season, the Chelsea loanee was forced to look elsewhere.
"I want to say a very big thank you to Schalke 04 for the time they gave me during an incredibly difficult period of my career," Baba said on social media.
"It's been an honour to have worn the colours of our great club.
"I've learned a lot both on and off the field that wants to go beyond football. Me and my family are grateful.
"I wish the club, the fans, the players, the medical team and the technical team all the best for the remaining season."
DEUTSCH. Liebe Fans, liebe Schalke Offiziellen, Liebe Mitspieler, Ich möchte mich bei Euch herzlich bedanken für die Riesenunterstützung während der schwierigsten Zeit meiner Karriere. Es war eine Ehre für mich die Farben dieses GROSSARTIGEN Vereins zu tragen. Ich habe sehr viel gelernt - sowohl auf dem Platz als auch abseits des Spielfeldes. Meine Familie und ich sind sehr dankbar. Ich wünsche dem Verein, den Fans, meinen Mitspielern, dem Ärzte- und Physioteam, dem Trainerteam und dem Management nur das allerbeste für die restliche Spielzeit. Viele Grüße aus Reims, Euer Baba🔵⚪️ ENGLISH. Hello SCHALKE fans I want to say a very big thank you to @S04 for the time they gave me during an incredibly difficult period of career. It’s been an honour to have worn the colours of our GREAT club. I have learnt a lot both on and off the field that will go beyond football. Me and my family are grateful. I wish the club, the fans, the players,the medical team and the technical team nothing but the best for the remaining season. BABA RAHMAN🔵⚪️
While at Schalke on an initial one-year loan from Chelsea, the defender suffered an injury while playing for Ghana at the 2017 Africa Cup of Nations.
Initially expected back in action after seven months, a series of complications kept him in the treatment room until May last year when he made a second-half appearance on the final matchday of the 2017-18 Bundesliga season, having rejoined the Royal Blues for a second spell on an 18-month deal.
His frustrations led to the termination of his deal and a move back to Chelsea, who, on Wednesday, sent him on another loan to Reims for the remainder of the season.